1D Solar Wind Simulations Using Cylindrical VPIC*

The temperature of the solar wind plasma expanding from the hot solar corona does not decrease with the distance as fast as predicted by the adiabatic expansion law. The non-adiabatic solar wind cooling is a long-standing problem of space plasma physics. In this work we study how weak Coulomb collisions affect the temperature scaling of the isotropic part of the solar wind electrons using Cylindrical VPIC simulations. Cylindrical VPIC is a particle-in-cell code that imposes a B ~ 1/r scaling and has the scattering rates as a free parameter which can be changed independently, hence suitable to perform this study. The isotropic electrons are trapped in a parallel electrostatic potential that holds them back from escaping away from source to ensure quasi-neutrality with the ions. The level of the trapped population is a result of two competing processes- particle influx from the streaming population due to pitch-angle scattering and particle losses due to energy diffusion. The electron temperature was observed to scale with the ratio ν_ee/ν_ei,  as suggested by the collisional model.